Glass compositions which are intended for use as protective covers for solar cells must have a number of optical characteristics. Such glass compositions must, inter alia, exhibit the following properties:
(i) good white light transmission with at least 80% of incident light being transmitted at 400 nm and 85% of incident light being transmitted at 450 nm (for a sample having a thickness in the range of 50 to 300 microns);
(ii) low ultra-violet light transmission i.e. high UV absorption at wavelengths below 320 nm, with less than 5% incident radiation being transmitted, more preferably less than 2% incident radiation being transmitted (for a sample having a thickness in the range of 50 to 300 microns);
(iii) absence of significant discolouration (also known as radiation stability) when exposed to solar radiation of space which comprises inter alia, ultra-violet light, low energy electrons, protons and X-rays.
Furthermore, such glass compositions must have a coefficient of linear expansion which is the same as, or close to, that of silicon. Borosilicate glasses are found to have coefficients of linear expansion which are of the desired value.
It is known that incorporation of cerium oxide in borosilicate glass compositions leads to a glass having the high ultra-violet absorption which is required of a glass composition which is to be used as a protective cover for a solar cell.
However, it is difficult to incorporate cerium oxide into the glass in relatively large quantities (i.e. above 2% by weight) owing to the fact that cerium oxide tends to induce phase separation in the glass and also tends to impart a yellow to brown colouration in the glass thereby impairing the high white light transmission of the glass.